Co-extrusion is useful for many applications, including inter-digitated pn junction lines, conductive gridlines for solar cells, electrodes for electrochemical devices, etc.
In order to meet the demand for low cost large-area semiconductors, micro-extrusion methods have been developed that include extruding a dopant bearing material (dopant ink) along with a sacrificial material (non-doping ink) onto the surface of a semiconductor substrate, and then heating the semiconductor substrate such that the dopant disposed in the dopant ink diffuses into the substrate to form the desired doped region or regions. In comparison to screen printing techniques, the extrusion of dopant material on the substrate provides superior control of the feature resolution of the doped regions, and facilitates deposition without contacting the substrate, thereby avoiding wafer breakage. Such fabrication techniques are disclosed, for example, in U.S. Patent Application No. 20080138456, which is incorporated herein by reference in its entirety.
FIGS. 19(A) and 19(B) are plan views showing a typical metallization pattern formed a conventional H-pattern solar cell 30.
As shown in FIG. 19(A), H-pattern solar cell 30 includes a semiconductor substrate 31 having an upper surface 32, and a series of closely spaced parallel metal fingers (“gridlines”) 34 that run substantially perpendicular to one or more bus bars 35, which gather current from gridlines 34. In a photovoltaic module, bus bars 35 become the points to which metal ribbon (not shown) is attached, typically by soldering, with the ribbon being used to electrically connect one cell to another. The desired geometry for bus bars 35 in an H-pattern cell is about 1 to 2 mm in width and about 0.005 to 0.20 mm in height. These very wide and thin dimensions (low aspect ratio) create a challenge for conventional extrusion printing. For reliability reasons, it is desirable to avoid making the extrusion nozzle too narrow (or short) in order to avoid clogging, particularly when one is printing a particle filled material such as the silver loaded ink that is used to metalize solar cells. Furthermore, die-swell, the tendency for the ink bead to expand after it exits the nozzle, causes further thickening of the wet printed line. For cost reasons, it is desirable to print no more silver to form bus bar 35 than is necessary for soldering. For throughput reasons, it is desirable to print the bus bar 35 as rapidly as possible, specifically at speeds in excess of 100 mm/second, which equates to producing tens of megawatts of product per printer per year. Referring to FIG. 19(B), back surface 36 of H-pattern solar cell 30 typically has a metallization structure consisting of solderable silver bus bar lines 39 and a broad area aluminum back surface field coating 36. Typically these two metallizations are deposited in two separate screen printing steps.
FIG. 20 illustrates a problem encountered in the production of conventional H-pattern solar cells 30 using conventional techniques. In particular, FIG. 20 shows a problem commonly arising in the extrusion printing of the front metallization of H-pattern solar cell 30, where weak adherence of each gridline 34 to surface 32 of substrate 31, particularly at endpoints 34A of each gridline 34, results in poor conduction and possible loss (detachment) of gridline 34.
FIG. 21 illustrates another problem commonly arising in the extrusion printing of the front metallization of conventional H-pattern solar cell 30 using screen printing techniques. As indicated in FIG. 21, in most conventional H-pattern solar cells, the frontside gridline pattern includes gridlines 34 and busbars 35 that are substantially co-planar. In the case of screen printed solar cells, this co-planarity is the result of the simultaneous printing of both busbars 35 and gridlines 34 through a single screen. In the case of dispensed gridlines in the prior art (for example U.S. Pat. No. 5,151,377 by Hanoka), low viscosity inks are employed with cause the bus bar and gridline vertex to reflow and substantially planarize. The coplanar structure is thought to be important in order to obtain a robust solder joint between the bus bar and the string ribbons that are attached to the bus bar. However, with these conventional methods, breaks in the busbars have been observed at the busbars/gridline intersection, making cell testing and sorting inaccurate because of an anomalously large series resistance.
What is needed is a system and method for producing H-pattern solar cells at a low cost that addresses the problems associated with conventional extrusion and screen printing manufacturing techniques described above, and is acceptable to the solar cell industry.